Particulate metallic zinc is produced commercially by two methods:
(1) controlled condensation of zinc vapor, and PA0 (2) atomization of molten zinc by impinging a pressurized fluid (generally compressed air) onto a stream of the molten zinc. PA0 (1) flow rate, PA0 (2) apparent density, PA0 (3) particle shape, and PA0 (4) particle size and size distribution.
The product of the first method, commonly referred to as "zinc dust", is quite small in particle size, almost all passing through a 325 mesh screen (U.S. Standard), is spherical in shape, and contains about 3-5% zinc oxide. The atomized zinc material, commonly referred to as "zinc powder", is larger in particle size, is irregular in particle shape, and typically contains less than 1.5% zinc oxide.
Included among the most important physical properties of particulate metals, more commonly referred to as metal powders, are:
These properties are closely related in that the apparent density, that is, the weight of a given volume of loose powder, depends not only on the material density but also on particle shape, size and size distribution. Zinc dusts with their small size, narrow size distribution and spherical shape exhibit apparent densities in the range of 3.0-3.5 g/cc. Zinc powders with their larger size, irregular shape and wider size distribution are also characterized by a low apparent range of 2.7-2.9 g/cc. Thus, the finer, spherical zinc dusts with a narrow size range and the coarser, irregular zinc powders with a broad size range exhibit practically the same apparent densities, neither exceeding about 3.5 g/cc.
Flow rate, that is, the time necessary for a specific weight of powder to flow through an orifice, is also dependent on particle size and shape. Free flow of powder is advantageous for rapid and complete filling of die cavities. Flow rate increases as the particle size increases, and for a given particle size, spherical particles flow faster than irregular particles.
While the particle size of the spherical zinc dust produced by condensation of zinc vapor may be varied to some extent, it nevertheless remains quite small and of narrow distribution. In any case, the generation of the required zinc vapor results in an energy consumption much greater than that of the atomization process wherein it is required that the zinc only be molten. By varying atomizing fluid pressure and molten zinc stream diameter, the particle size of the atomized zinc powder may be varied to a much greater extent than that of the condensed zinc dust. Unfortunately, the irregular particle shape persists.
Thousands of tons of each of these products are consumed each year. The more finely divided zinc dust finds its greatest application in protective coatings. Applications for zinc powders include primary alkaline batteries, friction materials, chemical formulations, mechanical plating, spray galvanizing and others. Up to the present time, only irregular zinc powders have been used in these applications because spherical zinc powders are not commercially available. It is known that the chemical and physical properties of zinc powders vary with particle shape and size. Irregular-shaped particles possess a greater surface area per unit volume, slower flow rate and a lower apparent density compared to spherical-shaped particles of equivalent particle size distribution. Efficient separation of a powder into its various particle size fractions by mechanical screening is more difficult with irregular particles. Thus, the characteristics of spherical zinc powders offer potential advantages in the above-mentioned applications.
For example, powders with spherical shape are commonly used in the fabrication of filters where controlled porosity is obtained through the use of carefully graded particle sizes. Similar control of porosity might be used advantageously in alkaline primary batteries where a more uniform porosity should provide a better distribution of the KOH battery electrolyte, possibly resulting in improved performance.
Air atomized zinc powder has been commercially produced since the early 1940's and various processes are known in the art for the preparation of same. Included amongst the most significant prior art in this area are the followng references:
U.S. Pat. No. 2,255,204 which issued on Sept. 9, 1941 to Best teaches an improved atomized brass powder, the particles of which are rounded and spherical, and which exhibits the characteristics of being both free-flowing and of relative high apparent specific gravity, produced by atomizing molten brass containing from about 0.05 to about 1% of phosphorous.
U.S. Pat. No. 3,041,672 which issued on July 3, 1962 to Lyle teaches a process for producing fine particles of spheroidal shape which utilizes the step of striking an arc between a stick electrode and a nozzle electrode, introducing a consumable wire or rod into the plasma effluent of the arc and passing a gas stream along the plasma to shear off molten material, thereby forming small droplets which are removed from the arc zone and solidified.
U.S. Pat. No. 3,293,334 which issued on Dec. 20, 1966 to Bylund et al. teaches a method of manufacturing fine spherical aluminum powder wherein the molten metal is atomized and the particle formed is initially typically elongated owing to the action of the use of high velocity gas which tears the particle away from the stream of molten metal as it issues from the atomizing nozzle.
This method employs the use of a high velocity stream of exothermic gas containing nitrogen and carbon dioxide to disintegrate the metal into fine particles and to provide available oxygen in an amount insufficient to interfere with surface tension forces of the particles affecting sphere formation and subsequently passing the gas stream along with the particles of molten metal through the spheroidizng zone to cause sphere formation and solidification.
U.S. Pat. No. 3,340,566 which issued on Sept. 12, 1967 to Woosley et al. is directed to an apparatus for the production of metal particles by atomizing the molten metal with an inert gas and subsequent exposure of the atomized metal to an inert gas providing controlled oxidation of the metal surface, and is particularly concerned with the production of spherical aluminum powders.
U.S. Pat. No. 3,420,289 which issued on Mar. 4, 1969 to Aikawa et al. teaches an apparatus for preparing high purity fine powder of low melting metals including zinc and is particularly concerned with the physical arrangement of the apparatus involved.
U.S. Pat. No. 3,501,802 which issued on Mar. 24, 1970 to Ullman et al. also teaches a method and apparatus for producing metal powders in which molten metal is atomized in a tower by a tornado like spiral stream of fluid produced by an atomizing nozzle device.
None of the foregoing teachings is particularly concerned with the improvement provided by the method of the present invention which is directed to the preparation of spherically shaped zinc powders using commercially available atomization techniques which have generally heretofore persisted in producing irregular shaped particles.
It is, therefore, an object of the present invention to provide for a method for the preparation of spherical zinc powder particles using commercially available atomizing techniques.
It is also an object of the present invention to provide for a method for producing spherical zinc powder particles which exhibit a spherical morphology over a wide range of particle sizes.
It is a further object of the present invention to provide for spherically shaped zinc powder particles having improved flow and higher apparent density characteristics as compared with irregular shaped zinc powders produced using conventional air atomizing techniques.
It is yet a further object of the present invention to provide for the production of an atomized spherical zinc powder which exhibits hydrogen gassing characteristics comparable to those of commercially available irregular zinc powders for use in alkaline batteries.
These and other objects of the invention will become apparent from the following description of the invention.